Asynchronous Access Research Articles

A high-resolution interval timer and buffered storage system for neurophysiological experiments

A high-resolution interval timer, designed as a computer peripheral for use in neurophysiological experiments, is described. With only minimal intervention by the controlling computer, this device detects, identifies and times the occurrence of discriminated neuronal discharges and changes in experimental status, and autonomously stores these encoded times in an on-board 4K×16 r.a.m. This device reduces the need for detailed software timing routines, while still permitting user selectible resolution (>1 to <100 μs/bit) and asynchronous access to the stored data by the controlling computer. The flexibility of this device and its ability to collect data autonomously make it extremely useful in a wide variety of sensory and behavioural neurophysiological experiments.

Optimal phases of maximal length sequences for asynchronous spread-spectrum multiplexing

Optimal phases are given for maximal length linear feedback shift-register sequences (m-sequences) of periods 31 and 63 which are to be employed as signature sequences in an asynchronous direct sequence spread-spectrum multiple access communications system. The performance measure considered is the average signal/noise ratio at the output of the correlation receiver. The data presented is applicable to various forms of direct sequence spread-spectrum modulation including binary phase-shift keying, offset quadriphase-shift keying, and minimum shift keying modulation.

Flow control issues in a PAD

A packet assembly/disassembly (PAD) facility standard has been adopted by the CCITT to provide asynchronous terminal access to X.25-based packet networks. A PAD translates the character-by-character flow at the terminal interface into packet flow at the network interface. The paper describes the various control procedures necessary for dataflow within a PAD to prevent it from being congested. Topics discussed include character flow, packet flow, break signal handling, packet sequences, error messages, control messages, and echo buffering.

Error Probability of Asynchronous Spread Spectrum Multiple Access Communication Systems

Several approaches for the evaluation of upper and lower bounds on error probability of asynchronous spread spectrum multiple access communication systems are presented. These bounds are obtained by utilizing an isomorphism theorem in the theory of moment spaces. From this theorem, we generate closed, compact, and convex bodies, where one of the coordinates represents error probability, while the other coordinate represents a generalized moment of the multiple access interference random variable. Derivations for the second moment, fourth moment, single exponential moment, and multiple exponential moment are given in terms of the partial cross correlations of the codes used in the system. Error bounds based on the use of these moments are obtained. By using a sufficient number of terms in the multiple exponential moment, upper and lower error bounds can be made arbitrarily tight. In that case, the error probability equals the multiple exponential moment of the multiple access interference random variable. An example using partial cross correlations based on codes generated from Gold's method is presented.

Multiple Access to a Hard-Limiting Communication-Satellite Repeater

This paper analyzes the communication capability of a hard-limiting satellite repeater when spread-spectrum signals are used for asynchronous access multiplexing. It derives formal results which indicate the most suitable system bandwidth and the resulting maximum number of simultaneous users as a function of the ratio of the received signal power (over the entire system bandwidth) from the satellite to the available noise power density at the ground station receiver. Equally important, the paper sets forth the assumptions and approximations necessary to achieve the formal results and examines their weaknesses.